1. Field of the Invention
The presently disclosed and claimed invention relates generally to compositions comprising encapsulation materials, and in particular, but not by way of limitation, to compositions comprising anionic lipids without a phosphate group, and methods of producing and using same.
2. Description of the Background Art
Vesicles encapsulating useful substances in the internal aqueous phase and their dispersions are an important technology in various fields such as pharmaceuticals, perfumes, cosmetics and food stuffs. Examples of widely used lipids that constitute the membrane of the vesicle/liposome include negatively charged (anionic) phospholipids. Anionic phospholipids have previously been used in liposomes for imparting size-stability, enhancing encapsulation of pharmaceuticals and/or to modulate pharmacokinetics and pharmacodynamics of liposomes. However, said anionic phospholipids have been shown to induce untoward reactions in biological systems, and the resultant toxicity has been manifested by severe side effects such as but not limited to, acute thrombocytopenia, complement activation, dysfunction of white blood cells and the like.
Safety and adequate availability of blood is still a major concern in transfusion medicine. The requirement of pre-transfusion processing, storage and cross-matching of blood are other factors that have given impetus to the search for safe, shelf-stable and efficacious oxygen carrying fluids. An oxygen carrying fluid mimicking red blood cells (RBCs), in efficacy and safety profile is the goal. An ideal oxygen carrier would be hemoglobin that is encapsulated and is supplemented with the oxido-reductive system of RBC. One approach to compartmentalize hemoglobin is to encapsulate hemoglobin in liposomes [Awasthi, 2005; Phillips et al., 1999; Takaori et al., 1996; Usuba et al., 1994; Sakai et al., 1993; Farmer et al., 1988]. This encapsulated product has been variably termed hemoglobin vesicles (HbV), neo-red cells (NRC) or liposome-encapsulated hemoglobin (LEH), and said product contains highly concentrated (>36 g/dl) purified hemoglobin within the phospholipid membranes. Hemoglobin vesicles or liposome-encapsulated hemoglobin (LEH) mimics membrane enclosed cellular structure of red blood cells [Phillips et al., 1999; Sakai et al., 1996; Rudolph, 1995]. Compared to free modified hemoglobin preparations, LEH is characterized by spatial isolation of hemoglobin by an oxygen permeable lipid layer that eliminates the toxicity associated with free modified or unmodified hemoglobin.
A major impediment in the development of LEH has been the low encapsulation efficiency of hemoglobin inside the vesicles. To increase the encapsulation of proteins inside liposomes, anionic lipids, such as dimyristoyl- and dipalmitoyl-phosphatidyl glycerol (DMPG and DPPG) are usually incorporated in the lipid composition [Drummond et al., 1999; Walde et al., 2001]. However, anionic liposomes rapidly interact with the biological system subsequent to their opsonization with complement and other circulating proteins [Miller et al., 1998; Szebeni, 1998]. Such an interaction has at least two acute consequences—a rapid uptake by the reticuloendothelial system (RES), and toxic effects, such as pseudoallergy that is manifested as vasoconstriction, pulmonary hypertension, dyspnea, drop in circulating platelets and leukocytes, etc. [Awasthi et al., 2007; Szebeni et al., 2000]. Since these reactions are mostly dependent on lipid dose, the problem is more challenging when huge quantities of liposomes need to be administered, such as in the use of LEH as a resuscitative fluid in acute blood loss. It is a challenge therefore, to encapsulate maximum amounts of hemoglobin in the least amount of lipid using anionic lipids and to keep the charge-associated toxicity in check.
Therefore, there is a need in the art for new and improved lipids and liposome encapsulation methods that increase encapsulation and stability while overcoming the untoward effects commonly seen with prior art encapsulation methods. It is to said lipid compositions and lipid nanostructures formed therefrom, as well as methods of producing and using same, that the presently disclosed and claimed invention is directed.